25-Hydroxy-cholecalciferol is used as an additive to animal feed and is available as Hy-DTM (ROCHE VITAMINS AG, Basel, Switzerland) to improve the health status of animals such as livestock and pets. In view of its physiological potency and the narrow therapeutic window dosaging of the compound is critical and therefore, reliable analytical means are required to monitor the amount of the compound in feed and its uniform distribution therein. Various methods for the quantitative determination of 25-hydroxycholecalciferol in plasma have been described which are based on immunoassays, see WO 99/67211 or on HPLC/mass spectrometry using derivatives or isotopes as internal standards, see Biological & Pharmaceutical Bulletin (2001), 24(7), 738-743. However, these known methods are not satisfying when applied to the analysis of feed samples.
As all the old methods show it is difficult to analyse 25-hydroxycholecalciferol in feed samples due to the presence of big quantities of solid chemical and biological substances, whereas plasma or serum consist mainly of water. Two types of methods are available. A physico-chemical method using HPLC and UV detection and an immunochemical method using HPLC for sample clean-up and radio-labeled immunoreagents, see Bruce. W. Hollis, Calcif. Tissue Int. (1996) 58:4-5. The other method, is also laborious and contains an analytical step, which uses radioactive material for the quantification. This method consists of the addition of 3H-25-hydroxycholecalciferol as internal standard, extraction with methanol, sample clean-up on reversed-phase SEP-PAK cartridges, further clean-up on normal-phase SEP-PAK cartridges, further clean-up on normal-phase HPLC and final intrinsic analytical reversed-phase HPLC. The overall recovery is determined by scintillation counting of the 3H-25-hydroxycholecalciferol. Quantification is done by external calibration and UV detection at 264 nm. The sample clean-up procedure is so laborious because the final quantification is done by UV. Such a complicated purification of the extract requires a determination of the recovery which is done using radio-labeled 25-hydroxycholecalciferol. Both methods are cumbersome, with many poor performance characteristics and reproducibility.